This invention relates to an integrated circuit (IC) for an active terminator adapted to be connected at an end of a bus line such as SCSI to make its high-speed operations possible and more particularly to such an IC capable of improving accuracy in the terminal resistance and reducing parasitic capacitance related to a probe test on the resistance value.
In the case of a bus adapted to transmit signals among a plurality of drive units, terminal resistors are connected individually to its lines, say, for preventing the "ringing." The resistance value of each terminal resistor is matched to the characteristic impedance of the line and a specified voltage is outputted to each line through its terminal resistor for making a high-speed operation possible. In order to avoid interference with another active terminator, such outputs of specified voltages may be carried out selectively in response to a control signal.
FIG. 5 shows a basic active terminator, considered and attempted earlier but not published by the assignee hereof (prior to the priority date herefor), formed as an integrated circuit 10, integrating such a circuit on a single chip, serving as a compact system allowing a plurality of drive units 41, 42, . . . to be connected through a bus. Thus, this active terminator IC is provided with a regulator 2 for receiving a source voltage V.sub.cc through a bonding pad 39 for external connection and generating a specified pull-up voltage V.sub.up, a capacitor C attached externally through another bonding pad 37 for smoothing the pull-up voltage V.sub.up and terminal resistors R and their bonding pads 31-3n each corresponding to one of a plural n-number of bus lines BUS1-BUSn. One end of each of the resistors R (corresponding to bus line BUSi where I is a dummy index 1-n) is connected to the corresponding one of the bonding pads 3i and the other end is connected through a switch SWi to the output terminal of the regulator 2. As each of the switches SWi is opened and closed in response to a control signal CTRL received from outside through a pad 38, the pull-up voltage V.sub.up may be outputted selectively to the bus line or may be put in a floating condition, depending on the situation.
FIG. 6 shows another active terminator IC 11, considered and attempted earlier but not yet published by the assignee hereof, for driving an amplifier having a plurality of pairs of a buffer amplifier and a terminal resistor corresponding to individual lines of a bus and being adapted to output a specified voltage to each line through the corresponding terminal resistor by the corresponding buffer amplifier. The capacitor C of FIG. 5 is not required in this case and, in order to improve the high-speed capability and isolation characteristics, the combination of regulator and switches in the case of FIG. 5 is replaced by a band-gap voltage source 2a for generating the pull-up voltage V.sub.up serving as a reference voltage. There are also provided n-number of buffer amplifiers 21-2n adapted to receive the output from the band-gap voltage source 2a and to current-amplify it. These buffer amplifiers (2i where I is again a dummy index 1-n) are connected such that the output of the amplifier 2i is transmitted through the corresponding terminal resistor R to the bonding pad 3i. As described in Japanese Patent Publications Tokkai 7-261890 and 7-240642, these buffer amplifiers 2i are also adapted to each receive a control signal CTRL through a bonding pad 39 such that the output condition can be selectably switched among a specified voltage, a floating condition and something else.
In order to keep the resistance value of each terminal resistor R within a specified range, each terminal resistor R is provided with an adjustment resistor such that the resistance value can be changed by switching on or off the connection. If the ideal resistance value is 100.OMEGA. but the resistance values of the terminal resistors R fluctuate between 99.OMEGA. and 101.OMEGA., for example, two adjustment resistors of 10 K .OMEGA. may be connected in parallel to a base resistor of 101.OMEGA. and a repair part 91 (or 92) which can be cut by a laser is inserted in the line of these adjustment resistors as shown in FIG. 7. If the resistance value of the terminal resistor R is 98.OMEGA. when measured after the end of preliminary processes at the time of production of the IC, both adjustment resistors are disconnected to make the resistance value equal to 100.OMEGA.. If it is 99.OMEGA., only one of them is disconnected and if it is 100.OMEGA., both adjustment resistors are left connected. Such process is carried out for each of the terminal resistors R.
When the internal resistance value of the IC is measured prior to the adjustment of the resistance values of the terminal resistors R, a probe test is carried out prior to the dicing process. In order to prevent the occurrence of measurement errors caused by variations in the contact resistance between the probe and the pad at the time of the probe test, it is necessary to provide voltage-detection pads in addition to current-forcing pads. For measuring the resistance value of a terminal resistor R with one end connected to a bonding pad 31, for example, this may be done as shown in FIG. 8 by providing a dummy pad 31a also connected to the end of the terminal resistor R like the bonding pad 31. Pads 71 and 71a are provided at the opposite end of the terminal resistor R such that, when the probe test is carried out, a constant current I is forced to pass from a probe pin 7a sequentially through the bonding pad 31, the terminal resistor R and the pad 71 to the other probe pin 7b and the voltage V1 at the pad 71a is detected by the probe pin 7c while the voltage V2 at the dummy pad 31a is detected by the probe pin 7d. The resistance value of the terminal resistor R is then obtained as (V2-V1)/I without being influenced by the contact resistance.
FIG. 9 shows still another active terminator IC 12, considered and attempted earlier but not yet published by the assignee hereof, having each of its bonding pads 31-3n provided with a dummy pad 31a-3na such that such a probe test can be carried out. Thus, the resistance value of the terminal resistor R corresponding, say, to the bus line BUS1 can be measured by using the bonding pad 31 as the pad for forcing the current I and the dummy pad 31a for the detection of voltage V2. The active terminator IC 12 of FIG. 9 is also provided with an extra bonding pad 30 not corresponding to any of the bus lines BUS1-BUSn and a voltage monitoring buffer amplifier 20 structured approximately identically to the buffer amplifiers 21-2n such that the pull-up voltage V.sub.up from the band-gap voltage source 2a can be monitored from outside through the buffer amplifier 20 and the bonding pad 30.
With such earlier considered active terminator ICs, a voltage-detecting dummy pad must thus be preliminarily set for each of the current-forcing bonding pads for measuring the resistance value of the terminal resistor. This causes the chip area to become large and since the parasitic capacitance increases, the speed of operating the bus is adversely affected. It is troublesome to measure the resistance value of each of many terminal resistors corresponding to the large number of lines in a bus and to make repairs on each, and it adversely affects the production cost.
In view of the above, it has been a common practice to put terminal resistors into use without carrying out measurements of resistance values or adjustments as long as it was allowed by the required degree of precision or the normal bus line specifications. If the level required by the specification was too severe, products with a large parasitic capacitance were put in use in spite of the expense, instead of carrying out measurements and adjustments on the terminal resistors.
With the increasing operating speed of multi-purpose CPUs, however, there is an increasing demand to increase the speed of bus lines and particularly device buses such as SCSI connecting to peripheral apparatus. In order to respond to such a demand, it is necessary to keep the resistance value of the terminal resistors even by sorting ICs and adjusting resistance values by measuring the resistance values of terminal resistors internally contained in the IC, reducing the parasitic capacitance and improving the high-speed characteristics of the bus operations. Since the limitations on the cost are also severe, the number of process steps, when the terminal resistors are measured and/or adjusted, must be reduced as stringently as possible.